Dual alloy turbine wheels or dual-property turbine disks have some limited use at the present time and are extremely attractive for future use in high performance commercial aircraft engine design. Single alloy turbine disks which are used predominantly in current technology commercial aircraft engines, are forged from vacuum melted ingots or are consolidated by various means from pre-alloyed powders. Such a single alloy must satisfy requirements in both the hub and the rim areas of the turbine disk which requirements are sometimes in conflict. The two extremes in single-alloy turbine engine disks today are the forged disks used in commercial and general aviation turbofan engines and the cast integral turbine wheel typically used in small turbo-prop/turbo-shaft engines and auxiliary power units. The forged alloys used today will typically have superior tensile and low cycle fatigue (LCF) properties, but quite limited creep rupture strength, while the cast wheel alloys will have the reversed properties, i.e. excellent creep rupture strength but relatively poor tensile and LCF properties. Modern turbofan engines, developing a thrust from 3,000 to 55,000 pounds and having cooled separately bladed turbine disks, require a turbine disk hub having maximized tensile strength in order to provide a satisfactory burst margin. The hub area must also have maximized resistance to low cycle fatigue (LCF) cracking and crack propagation in order to ensure long turbine disk life. The hub area must also have good notch ductility to minimize the harmful effects of stress concentrations, either inherent in the design or induced by undetected flaws in critical regions. In general, all the desirable qualities for disk hubs are associated with tough, fine-grained, highly-alloyed materials. In contrast to the hub, tensile stress levels are lower in the ring or rim of a well designed turbine disk, but operating temperatures are higher and creep resistance becomes an important consideration. With the current single alloy disk design philosophy, used for modern commercial aircraft and general aviation engines, the material is chosen primarily to satisfy hub requirements and sufficient cooling air is supplied to the rim to lower its temperature to the level, typically about 600.degree.-700.degree. C., where creep strength of the material is not limiting. If temperatures and stresses rise to levels where creep strength becomes limiting in the rims, large-grained alloys with adequate creep-resistance are employed, but the wheel size and weight are increased, since the large-grained creep-resistance micro structures have inferior tensile properties to fine-grained material.
Hence, from the above it is readily apparent that a dual property turbine disk becomes quite attractive as optimum properties in each area of the disk will allow the cooling air requirements for the disk to be minimized or eliminated, with resulting improvements in engine-operating efficiency. In addition, lighter weight turbine disks, would be possible with a favorable impact on total aircraft performance.
A dual alloy turbine disk which provides optimum properties for both the rim and the hub locations, will also permit superior low cycle fatigue cracking resistance in each area and will contribute to long life components that will reduce repair costs.
The dual alloy turbine disk concept is desirable for both separately bladed disk designs and also integrally-bladed turbine stages as used in small aircraft engines, which are currently made from a single piece casting. These small gas turbine engines are presently used in executive and business jet turboprop applications but are also receiving consideration for replacement of the current reciprocating engines used in the general aviation market.